86 BACTERIOLOGY. 



is provided with a cover of exactly the same design, but 

 of a little larger diameter. This cover, when placed 

 upon the dish containing the plates, fits over it and 

 prevents the access of dust. Prior to using, the dish and 

 cover should have been disinfected for one-half hour with 

 1:1000 sublimate, and then all the sublimate solution 

 allowed to drain from it. 



Into the bottom of this dish is sometimes placed a disc 

 of sterilized filter-paper moistened with sterilized water, 

 which serves to prevent the drying of the plates. This, 

 however, is not necessary. 



If agar-agar is employed, the dish and its contents may 

 be placed at a temperature of 37-38 C. ; if gelatin, 

 the temperature at which the plates are now to be kept 

 should not be over 22 C., otherwise the gelatin becomes 

 liquefied and the plates are rendered useless. 



When development has occurred, the object of the 

 dilution will easily be seen, and the different species of 

 bacteria in the mixture will be recognized by differences 

 in the character of the colonies growing from them. 



This, in short, is the plate method of Koch for the 

 separation of the individual species contained in a mix- 

 ture of bacteria. Many modifications of this method 

 exist ; all, however, are based upon the same principles. 

 The modifications have for their object the accomplish- 

 ment of the same end, but with a smaller armamenta- 

 rium of apparatus. 



PETRI'S MODIFICATION OF THE PLATE METHOD. 

 The modification which approaches nearest to the original 

 method, and at the same time lessens very materially the 

 number of steps in the process, is that suggested by Petri. 

 It consists in substituting for the plates small, round, 

 double glass dishes, which have about the same surface- 



